Geochemistry of the rare-earth elements and uranium in the acidic Berkeley Pit lake, Butte, Montana

Filtered (0.45 μm) and nonfiltered concentrations of rare-earth elements (REE), U, Zr, Th, Ba, Sc, and Y were measured as a function of depth in the Berkeley Pit lake, a large acidic mining lake in Butte, MT. The REE show very little variation with depth, apart from a slight concentration near the surface, presumably due to evaporation. The REE profiles of the pit waters show a depletion in light REE when normalized against NASC and the host Butte Quartz Monzonite (BQM), and a possible enrichment in middle REE vs. heavy REE when normalized against BQM. All of the REE partitioned weakly into secondary ferric precipitates formed by aging of deep Berkeley Pit water. The measured distribution coefficients increased across the lanthanide series from La (Kd=7.7) to Nd (Kd=10.3), and then decreased steadily to Lu (Kd=1.4). Under the conditions of the Berkeley Pit lake, the aqueous speciation of REE and U is dominated by sulfate complexes. Because the stability constants of REE–sulfate complexes show very little variation across the lanthanide series, the observed trends in Kd cannot be explained by aqueous complexation. Despite high concentrations of dissolved REE (e.g., 1.1 mg/l Ce) and U (0.85 mg/l), saturation indices for all solid phases were strongly negative, due to the low pH of the Berkeley Pit lake (2.3–2.6). The mobility of REE and U is more likely constrained by adsorption or co-precipitation with strengite, jarosite, schwertmannite, or other secondary minerals forming in the lake.